C++ String Unable to Read Memory

Question

I have a class that looks like the following:

class ModelCommand {
public:
    virtual ~ModelCommand() {};
};

class FolderCommand : public ModelCommand {
public:
    std::string text;
    unsigned width;
    bool isBackspace;

    FolderCommand(bool isBackspace, unsigned width, std::string text = "") : text(text), width(width), isBackspace(isBackspace) {}
};

class CModel {
private:
    string _folder;
public:
    void Update(std::shared_ptr<ModelCommand> &cmd);
};

Then in my controller I have an instance of my model and update it using the new FolderCommand object I create:

shared_ptr<CModel> model;

shared_ptr<ModelCommand> cmd = dynamic_pointer_cast<ModelCommand>(make_shared<FolderCommand>(false, 20, "a"));

model->Update(cmd);

And then inside my update method of CModel I try to do the following:

void CModel::Update(std::shared_ptr<ModelCommand> &cmd) {

    if (auto folderCmd = dynamic_pointer_cast<FolderCommand>(cmd)) {
        if(!folderCmd->isBackspace)

            // This is where _folder is unable to read memory
            _folder += folderCmd->text;

        else if(folderCmd->isBackspace && _folder.length() > 0)
            _folder.erase(--_folder.end());

        folderCmd->text = _folder;
    }
}

This results in the CModel's _folder variable being "Unable to Read Memory".

Can someone explain and provide a solution to this problem?

Thanks.

UPDATE Added some more code for clarification

Do you initialize the _folder member to something, before appending to it? — Theodoros Chatzigiannakis
– Theodoros Chatzigiannakis, Commented Apr 13, 2016 at 18:57
Yes I've tried initializing _folder to an empty string when the CModel gets constructed it resulted in the same error. — Booster
– Booster, Commented Apr 13, 2016 at 18:59
and the instanciation of FolderCommand, what are you passing to the constructor exactly — Ceros
– Ceros, Commented Apr 13, 2016 at 19:04

Carafini · Accepted Answer · 2016-04-13 19:23:47Z

3

Using the first snippet you posted, I called the Update method as below, and no "Unable to Read Memory" issue arose.

int main()
{
  shared_ptr<ModelCommand> fc(new FolderCommand("somestring\\"));
  CModel mod;

  mod.Update(fc);
  mod.print_folder();

  return 0;
}

I think that would be interesting to see how the Update method is being used.

Obs.: I added a public member function, print_folder, to print ou the _folder private member. Also, I'd make just a comment, however I still don't have this privilege.

answered Apr 13, 2016 at 19:23

Carafini

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5 Comments

Ceros Over a year ago

I believe this should be a comment and not an answer.

Carafini Over a year ago

Yep, I pointed that out. I just didn't have the privilege by then.

Booster Over a year ago

In my current code I used a shared_ptr to my model. When I looked at this example I simply made an instance of my CModel and called Update using that. As to why using a shared_ptr to my model resulted in an error I'm unsure but thanks Adriano Carafini!!

Ceros Over a year ago

@Booster you did not instanciate CModel in your example.

Booster Over a year ago

.. That would definitely do it.

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